Field of the Invention
The present invention relates to an SAW identification or sensor configuration operating with surface acoustic waves, for identification or sensor systems, in which an interrogation/evaluation device interrogates identification or sensor configurations with a wirelessly transmitted interrogation signal and evaluates a response signal sent back from the configuration, including acousto-electrically active components on a piezoelectric substrate.
Identification marks, which are referred to as ID tags for short in the following text, are identification configurations of the generic type and are used in identification systems for the non-contact automatic identification of objects. They are disclosed, for example, in German Published, Non-Prosecuted Patent Application DE 44 05 647 A1, corresponding to U.S. patent application Ser. No. 08/701,492, filed Aug. 22, 1996. Such ID tags are components in which an electrical signal is converted through the use of a transducer into a surface acoustic wave that is reflected on a sequence of reflectors. The reflected surface acoustic wave is converted back to an electrical signal by a transducer which may be identical to the transducer that converts the electrical input signal. Depending on the configuration of the reflectors, a predetermined code is produced which represents the ID tag. The electrical signal which represents the code is sent back to a system which emits the interrogation signal, as a result of which it is possible to identify the point at which the ID tag is disposed. Such identification systems can be used in a wide range of applications.
As is stated above, the information content of the response signal coming from the ID tag is determined by the configuration of the reflectors contained in it. The information may be defined, for example, in binary form by providing a reflector configuration for a predetermined number of information bits, in which configuration a 1-bit corresponds to an existing reflector and a 0-bit corresponds to a missing reflector, that is to say reflection back to the output transducer takes place on an existing reflector, while no bit produced by reflection is present in the case of a missing reflector. Such a coding corresponds to amplitude modulation.
The information content can also correspond to phase modulation if, in a sequence of reflectors, the individual reflectors are shifted from a predetermined position, which corresponds to a 0-phase angle or relation.
For example, SAW sensors which are disclosed in European Patent 0 619 906 and can be interrogated wirelessly are sensor configurations of the generic type. They contain first SAW structures as sensor elements and second SAW structures as reference elements. It is possible to interrogate measurements which are formed by comparing the output signals from the sensor and reference elements in response to interrogation signals.
However, configurations of the type explained above must be physically constructed in such a way that pulses in the response or measurement signal can be reliably separated in order to ensure that it is possible to resolve the information in the response signal reliably. Since the resolvable time pulse interval or bit interval is inversely proportional to the bandwidth, if the available bandwidth of the interrogation signal is small, that results in a long time bit interval, from which it follows, in turn, that the distance between the SAW structures in identification or sensor configurations must be correspondingly large. Particularly if the SAW structures are disposed in a single acoustic track, that requires a correspondingly long piezoelectric substrate for the configuration. That in turn will result in a high level of physical complexity, especially for SAW configuration housings.